This invention relates generally to a method and apparatus for surface treatment by plasma, and more particularly to apparatuses for plasma etching and plasma deposition (or plasma CVD (chemical vapor deposition)) for semiconductor integrated circuits.
Surface treatment techniques using plasmas are used actively and widely. A plasma surface treatment apparatus usually comprises a vacuum chamber, means for evacuating the vacuum chamber, means for introducing a gas into the vacuum chamber, means for generating plasma within at least part of the vacuum chamber, a specimen (or substrate), and means for holding the specimen. The characteristics of plasma surface treatments vary widely with the type, composition, and concentration of the plasma-generating gas (discharge gas). Depending upon the object of the surface treatment, it is sometimes necessary to enhance specific treatment characteristics by changing at predetermined time intervals (periodically) the type, composition, and concentration of the discharge gas during the surface treatment. In prior-art apparatuses, however, no function for changing the type, composition, or concentration of the gas is provided for the gas introduction means thereof, so that only a surface treatment with predetermined characteristics can be carried out throughout the surface treatment process.
FIG. 1 illustrates a typical construction of a conventional etching apparatus using plasma. The apparatus shown in FIG. 1 is an apparatus using magnet-microwave discharge, and FIG. 2 illustrates a method using RF (radio-frequency) discharge. Means for generating the magnet-microwave discharge consists of a magnetron 1, a magnetron power supply 2, a waveguide 3, a discharge tube 4, solenoids 5 and a permanent magnet 12. The solenoids 5 and the permanent magnet 12 are both not always necessary, and either one may be used alone. Means for generating the RF discharge consists of an RF power supply 15, a capacitor 16 and upper and lower RF electrodes 13 and 14. Although the RF electrodes 13 and 14 are shown within the vacuum chamber 6 in FIG. 2, they are sometimes arranged outside the vacuum chamber 6.
The following are important factors in the application of a plasma-etching apparatus to the production of a semiconductor device:
(1) etching speed is high; and PA0 (2) a material 24 being etched, shown in FIG. 3(a), can be etched vertically, as shown in FIG. 3(c), without any undercutting, using a mask 25, that is, a fine pattern fabrication is possible.
The etching rate can be improved by using SF.sub.6 or F.sub.2 as the discharge gas when the material 24 being etched is Si (or polysilicon), for example. Since this discharge gas results in extensive undercutting 26, as shown in FIG. 3(b), however, condition (2) above cannot be met thereby. Conditions (1) and (2) can be satisfied simultaneously by periodically changing the composition of the discharge gas in accordance with the present invention, as will be described later.
The apparatuses shown in FIGS. 1 and 2 can be used as plasma CVD (chemical vapor deposition) apparatuses by changing the kind of discharge gas used. When a mixed gas of SiH.sub.4 and NH.sub.3 is used as the discharge gas in one of the apparatuses of FIGS. 1 and 2, for example, a film of a mixture of Si and N (silicon nitride; Si--N film) is formed on the specimen surface, and this film can be used as a protective film for the resultant semiconductor device. However, when a large quantity (at least 10%, atomic density ratio) of hydrogen is mixed into this silicon nitride film, the device characteristics is reduced [R. B. Fair et al: IEEE, ED-28, 83-94 (1981)]. When a mixed gas of SiF.sub.4 and N.sub.2 is used as the discharge gas in the apparatus of FIG. 1, silicon nitride can be formed similarly. In this case, however, the fluorine mixed into the film raises a problem. The present invention can greatly reduce the quantity of the hydrogen or fluorine mixed therein by changing periodically the composition of the discharge gas.